输气钢管内壁椭圆形坑失效原因分析

为分析某输气用X52QS钢级无缝钢管内壁椭圆形凹坑失效原因，通过力学性能测试、金相检测、化学成分分析、扫描电镜（SEM）、物相（XRD）分析等手段，对该凹坑产生原因进行综合分析。结果发现，凹坑区的C、Cr、Mo、Ti、B等元素含量高于正常区，且偏聚在凹坑区；C、B元素含量超标，化学成分不均导致凹坑区金相组织不均匀，使凹坑区发生微观原电池反应，凹坑区成为阳极被腐蚀而减薄；此外，凹坑内表面的CaCO3和SiO2含量较高，造成凹坑区垢下腐蚀；凹坑区表面膜疏松造成浓差电池，在介质冲刷等共同作用下会加速腐蚀，凹坑区壁厚持续减薄，最终形成更大的椭圆形凹坑。最后对样管最薄凹坑剩余厚度进行计算，发现已无法满足设计压力要求，建议换钢管或者降低设计压力后二次利用。

Failure Analysis of Oval Pits on Inner Wall of Steel Pipe for Gas Transmission

In order to analyze the failure reason of the oval pit on the inner wall of a X52QS steel seamless steel pipe for gas transmission, the causes of the pit were comprehensively analyzed by means of mechanical property test, metallographic examination, chemical composition analysis, scanning electron microscope (SEM) and phase (XRD) analysis. The results showed that the content of C, Cr, Mo, Ti, B and other elements in the pit area was higher than that in the normal area and concentrated in the pit area. The content of elements C and B exceeds the standard, and the uneven chemical composition leads to the uneven metallographic structure in the pit area, which causes the microscopic galvanic cell reaction in the pit area, and the pit area becomes the anode which is corroded and thinned. In addition, the content of CaCO3and SiO2 on the inner surface of the pit is high, causing corrosion under the scale in the pit area. The loose facial mask on the surface of the pit area causes the concentration cell, which will accelerate the corrosion under the combined action of medium scouring, and the wall thickness of the pit area will continue to decrease, eventually forming a larger oval pit. Finally, the remaining thickness of the thinnest pit of the sample pipe is calculated, and it is found that it cannot meet the design pressure requirements. It is recommended to replace the steel pipe or reduce the design pressure and reuse it.